The present invention relates to a fluidized bed combustion system that can be operated or controlled under pressure and includes a combustion chamber, a flue gas line leading therefrom in which is disposed a dust separator, an air compressor that is disposed in a line for air for combustion and is connected on the same shaft as an expansion turbine that is disposed in the flue gas line, with at least one controllable line, for returning a cooled portion of flue gas to the combustion chamber, branching off from the flue gas line downstream of the dust separator.
U.S. Pat. No. 4,705,433 discloses a fluidized bed combustion system of this general type where the returned flue gas is used as an inert transport gas for the fuel that is to be burned in the fluidized bed combustion system. Disposed in the combustion chamber is a heat-transfer surface assembly for withdrawing heat from the combustion chamber. The flue gas that is not used to transport fuel is expanded in an expansion turbine that drives an air compressor.
German Offenlegungsschrift 35 36 451 discloses a fluidized bed combustion system whereby during operation flue gas at a high temperature, preferably the combustion chamber temperature, is fed through an expansion turbine that on the same shaft is connected via at least one coupling to an air compressor that is disposed in an air line and to an electrical machine. In this connection, the electrical machine can be operated as a drive motor for the air compressor, or as a generator. This type of combustion system is always used in a combination gas and steam turbine process because the heat released in the combustion chamber is proportionately used in an arrangement that comprises several heat-transfer surfaces for generating steam and for heating the withdrawn flue gases.
In order to also be able to adjust the reaction temperature of about 850.degree. C. that generally prevails in the fluidized bed with a view to the formation of environmental pollutants in the flue gas, such as NO.sub.x, the combustion chamber of the fluidized bed combustion system is tied into the water/steam circulation of the steam generation. Due to a relatively high flue dust content in the flue gas in conjunction with a high reaction temperature in the combustion chamber of the fluidized bed combustion system, the heat exchangers and heat-transfer surfaces required for this are greatly affected by erosion and abrasion.
It is an object of the present invention to design a fluidized bed combustion system that can operate or be controlled under pressure in such a way that no heat exchangers and heat-transfer surfaces are disposed in the combustion chamber of the fluidized bed combustion system, yet the reaction conditions required for a minimum formation of pollutants are maintained.